Aluminum hydroxide has been conventionally used as a filler to be filled into a resin such as rubber or plastics in a wide variety of applications. One of the major applications out of them is the use of aluminum hydroxide as a flame retardant filler. In addition, aluminum hydroxide has been recently used also as a filler for a thermally conductive sheet for discharging heat from, for example, such a large-scale glass substrate as to enable the formation of a flat panel display such as a plasma display panel (PDP).
When aluminum hydroxide is used as a flame retardant or a heat-discharging filler, an increase in amount in which aluminum hydroxide is filled is desired in order that aluminum hydroxide may sufficiently attain its object. However, increasing the amount in which aluminum hydroxide is filled typically causes such a problem as described below. An increase in viscosity occurs, and hence a kneading torque upon production of a molded article increases to deteriorate moldability.
In view of the foregoing, the following technology has been reported (see Patent Document 1). Two or three aluminum hydroxide powders having secondary particle diameters and specific surface areas within specific ranges are blended so that a reduction in viscosity upon filling into a resin may be achieved and an increase in amount in which aluminum hydroxide is filled may be enabled. That is, the technology aims at a reducing effect on the viscosity based on the embedment of middle particles (10 to 15 μm) or fine particles (0.5 to 10 μm) in gaps between coarse particles (35 to 150 μm) or the middle particles by mixing aluminum hydroxide powders containing the coarse particles, the middle particles, and the fine particles to form a powder having a wide particle size distribution.
Although the above-mentioned technology can suppress an increase in viscosity to enable an increase in amount in which aluminum hydroxide is filled, it cannot be said that sufficient investigations have been conducted on the strength properties of the resultant molded article. That is, the aluminum hydroxide powder requires the blending of at least the powders containing the coarse particles (35 to 150 μm) and the fine particles (0.5 to 10 μm), and forms a wide particle size distribution, but the presence of, in particular, such coarse particles as described above may cause the following trouble. When a molded article is obtained by filling the powder into a resin, a site where a stress is apt to converge is formed, and the strength (bending strength) of the molded article reduces. Accordingly, the powder is not suitable for use as, for example, a flame retardant in an electric-related sealing material or the like because flame retardant properties are obtained but the strength becomes insufficient.    Patent Document 1: JP 2002-114867 A